Inhibition of brain cation pump enzyme by in vitro lead ion: effects of low level [Pb] and modulation by homogenate

Rat brain homogenates were preincubated with lead chloride for 20 min at 0 degrees C. Inhibition of K-pnitrophenylphosphatase (K-pNPPase), which measures the dephosphorylation step of Na,K-ATPase, reached steady state within 10 min and was readily reversible. The activity in brain, homogenates prepared from 60-day-old rats, expressed (mean +/- SE) as a percentage of control (100.0 +/- 0.48%), fell to 89.4 +/- 0.52% at 0.25 microM [Pb] to 64.2 +/- 1.65% at 5 microM [Pb]. However, between 5 and 25 microM [Pb] K-pNPPase activity significantly increased (to 85.4 +/- 3.14% at 25 microM) before it again fell above 30 microM. Similar results were obtained with brain homogenates prepared from 10-day-old rats. The multiphasic nature of this dose-response curve did not change with changes in buffer, substrate, anion, test tube order, or test tube composition (glass vs plastic), and appeared to be intrinsic to homogenate-Pb interactions. Microsomal preparations also exhibited multiphasicity, but with a shift in the inflection points. However, there was no multiphasicity with commercial hog brain Na,K-ATPase, suggesting an interaction between a component in the rat brain preparations and lead ion. Addition of boiled rat brain homogenate to commercial Na,K-ATPase resulted in partial protection from lead inhibition but did not produce multiphasicity over the ranges tested. Bovine serum albumin provided less protection. We conclude that there is a complex interaction among K-pNPPase activity, some factor in the rat brain homogenate, and low levels of lead ion.